1. Field of the Invention
The invention relates to a pyrotechnical driving device, especially for a back-tightening device of a safety belt system, with a cartridge containing a propellant charge, preferably a solid propellant charge which is connected to a cylinder which contains a driving piston and is separated from the charge by a bursting plate which is ruptured after the propellant charge is activated and accelerates the driving piston.
2. Description of the Prior Art
A cartridge containing a propulsion charge is connected tightly to a cylinder, for instance to an elongated cylinder tube, and contains a piston which can be moved axially along the cylinder. If a curved cylinder is used, a spherical piston is guided in the cylinder. The space containing the propulsion charge is separated from the cylindrical space by a bursting bottom which may be integrally connected to the cartridge, or a disc-like cross section, which after the propulsion charge has been fired and at the predetermined pressure is ruptured, whereupon the pressure can act unimpeded on the piston. The cartridge is preferably made of a metallic material. When the bursting plate is ruptured, its effect is to cause a large pressure drop to take plate immediately. Because of this large pressure drop complete combustion of the solid propellant is not effected and complete chemical conversion of the energy contained in the unactivated propellant into acceleration energy for the piston does not occur. The possibility exists of making the rupture plate much thicker so that it is ruptured only at a much higher pressure. However, in this case considerable energy is used for the rupture energy which then is no longer available for the driving power of the pyrotechnical driving device.